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The Southbridge

Beginner's Guide to Motherboard Selection
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The Southbridge contains most peripheral, multimedia, and communications busses, including the PCI controller (Peripheral Components Interconnect), ATA controller (for hard drives and optical drives), USB controller (Universal Serial Bus for external devices), network controller interface, audio controller interface, and often even a modem interface. Most chipsets from any given time period offer similar performance, but reviews can inform the buyer of potential inadequacies, such as which recent Southbridge designs had the worst Serial ATA performance, and which had sub-par USB performance.

In the face of stiff competition, the ATA controller represents an opportunity for manufacturers to differentiate themselves not necessarily by improved performance, but by improved features. All manufacturers now offer RAID mode for Serial ATA controllers, which allows the user to run up to four drives in a secure or high-performance array. Nvidia went one step up by allowing Serial ATA and UltraATA hard drives to be mixed in an array. The latest improvisation is Intel Matrix RAID, which offers two different RAID arrays across only two drives at the same time.

Gigabit networking is now mature, and most chipsets include dedicated links to a Gigabit PHY (physical chip, which takes care of the physical connection). High-end chipsets typically feature two links for two gigabit connections, though midrange motherboards often lack the second PHY. The latest features from Nvidia are teaming, packet sorting and TCP/IP acceleration, characteristics formerly available only in high-end routers and discrete network cards.

The former MCP-T Southbridge from Nvidia set the previous high-mark for integrated audio by including a digital audio processor. Most high-end Southbridge designs now feature Azalia High Definition audio, and rely on an HD-capable codec chip to split the digital signal into eight analog channels. Azalia solutions do not include real-time Dolby Digital encoding for outputting system-generated surround sound (such as game surrounds) to an S/P-DIF connector, but even Creative's high-market X-Fi discrete sound cards lack this feature.

One Southbridge feature available to all manufacturers but not often used is a PCI Express hub. Instead, designers usually place all desired PCI Express lanes on the Northbridge and limit the number of supported slots. The nForce 590 series differs from those designs by placing a complete PCI Express controller on both north and south bridges and linking those bridges by fast HyperTransport links, so that one Northbridge is able to serve motherboards with as many as 48 or as few as 20 lanes.

Limiting motherboard selection by first choosing a Northbridge further limits choices in Southbridge, as most chipsets only support a small number of Northbridge/Southbridge combinations from the same chipset manufacturer. The exceptions use AMD's HyperTransport protocol to link chipset components. These come from Nvidia, ATI and ULi, but brand-different combinations are likely to stop since Nvidia has acquired ULi.

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